1. Field of the Invention
This invention relates to a cross-linkable rotational molding composition, to a method of rotational molding and to rotational molded articles. More particularly, it relates to an ethylene homopolymer or copolymer composition which can be cross-linked during rotational molding. This invention especially relates to a cross-linkable and rotationally moldable ethylene homopolymer or copolymer prepared in the presence of a particular solid polymerization catalyst complex.
2. Description of the Prior Art
Ethylene homopolymer and copolymers are fabricated into useful articles by a variety of techniques including blow molding, injection molding and rotational molding. Rotational molding (also called rotomolding) can be employed to produce hollow items of almost any size, open or closed and of any desired shape. Rotational molding techniques have been highly developed in recent years so that the production of many items by rotational molding can compete with those produced by blow molding, injection molding and thermoforming. In many cases, articles virtually impossible to fabricate by any of these other techniques can be produced by rotational molding. The main differences between rotational molding and other molding techniques, such as blow and injection molding, are (a) the use of resin powder instead of pellets, (b) the fact that the resin melts in the molds instead of being forced under pressure into the molds in a molten state, (c) biaxial rotation of the mold, and (d) the comparatively very low mold cost. Rotational molding molds are inexpensive because of their simplicity due to the lack of pressure in processing and of coring for water cooling.
In rotational molding, a charge of powdered thermoplastic material is placed inside a hollow mold. The mold and charge are rotated on two axes at predetermined speed ratios while the whole is heated to the melting point of the plastic. The melted plastic is distributed throughout the mold to produce a complete article. The mold is then cooled, solidifying the plastic and finally, the molded part is unloaded.
All thermoplastic resins can be used in rotational molding; polyethylene is the most widely used polymer but others including ethylene copolymers such as ethylene-vinyl acetate are gaining in use. U.S. Pat. No. 3,974,114 of Sowa discloses that additions of fatty acid salts, such as calcium or lithium stearate, to olefin polymer-based rotational molding compositions provide rotational moldings with pinhole-free surfaces. U.S. Pat. No. 3,627,869 of Walton discloses a method of producing a rotational molded product having good environmental stress crack resistance (ESCR) by forming a laminate of an ethylene homopolymer or copolymer and a copolymer of ethylene and propylene. U.S. Pat. Nos. 3,876,613 of Needham et al. and 4,029,729 of Rees et al. teach that ethylene polymers can be cross-linked during rotational molding by adding effective amounts of the acetylenic diperoxy hexynes or octynes of U.S. Pat. No. 3,214,422 of Mageli et al. The molding compositions disclosed by Needham et al additionally include an antioxidant of an ester of thiodipropionic acids which does not have a negative effect on the low temperature impact strength of the molded article as do other antioxidants. Both Needham et al. and Rees et al. employ ethylene polymers having a melt index of at least about 10 prepared by any suitable method including solution polymerization with chromium oxide-based catalysts. A useful rotational molding composition is disclosed in U.S. Pat. Nos. 4,032,600 and 4,185,067 of MacAdams et al which is said to provide molded articles of high impact resistance, high stress crack resistance, with smooth surfaces and an absence of excessive warpage. This composition is a blend of three polymers: high density ethylene homopolymer or copolymer, a random copolymer or terpolymer of ethylene and propylene and a copolymer of ethylene and vinyl acetate.
Most cross-linkable ethylene polymers employed heretofore in commercial rotational molding compositions have been prepared by solution polymerization techniques. Although ethylene polymers prepared by particle form polymerization are available and are less costly than those prepared by solution polymerization they have not been available heretofore in a form whereby they may be cross-linked during rotational molding.
It is an object of this invention to provide a crosslinkable rotomolding composition utilizing a particle form ethylene polymer.
It is another object of this invention to provide a method of rotational molding to produce an article of a cross-linked particle form ethylene polymer.
It is a further object of this invention to provide a rotomolded article of a cross-linked ethylene polymer prepared by particle form polymerization.
The achievement of these and other objects will be apparent from the following description of the subject invention.